BMI and obesity: the implications of a body mass index over 30

The word “obesity” is thrown around often in discussions of weight. Most of us think we have a vague idea of the definition of the word (or perhaps we feel we know it when we see it). But the truth is, no one can assess another person’s health simply by looking at them. So, who—or what—decides how a number on the scale classifies a person's weight?

While obesity might seem like an arbitrary concept based on personal views, there is a medically accepted screening tool to determine where a person falls on the healthy weight spectrum: BMI. 

However, BMI is not without problems. The screening tool has many shortcomings and is backed by research that leaves much to be desired. If you've been told that your BMI score falls within the obese category, but aren't sure what that means, continue reading to learn more about BMI and obesity.

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BMI—short for body mass index—is a quick-and-easy screening tool often used by healthcare providers to classify obesity in children, teenagers, and adults.

Your BMI is calculated with a formula using only your height and weight—meaning it doesn't take factors like muscle mass, age, biological sex, ethnic/racial background, or lifestyle into account. Additionally, the research data used to back BMI as a screening tool is largely from white populations. For these reasons, BMI is an imperfect assessment of health as it pertains to weight.

So how does it work? BMI scores are broken down into four main categories: underweight, normal, overweight, and obese. A normal—or healthy—BMI is anything that falls between a score of 18.5 and 24.9. An underweight BMI is 18.5 and under, and an overweight BMI is 25 to 29.9.

According to the National Institutes of Health (NIH), more than two out of every five people in the U.S. have obesity, meaning they have a BMI of 30 or above (NIH, 2021).

BMIs in the obese range are broken down further into three separate classes:

• Class 1: BMI is 30 to 34.9

• Class 2: BMI is 35 to 39.9

• Class 3: BMI is 40 or above

Class 3 obesity is sometimes referred to as morbid, extreme, or severe obesity.

Having a high BMI is related to a number of serious—and potentially fatal—health problems. However, it’s important to note that these factors are actually associated with high body fat, not necessarily the BMI itself.

Heart disease

Heart disease is a catch-all term that refers to all diseases that affect the heart, like peripheral artery disease, coronary artery disease (which causes heart attacks), and arrhythmia. Having a high BMI can increase your risk of developing heart disease (Khan, 2018).

Stroke

Strokes happen when the brain's blood supply is cut off, resulting in brain damage, disability, weakened muscles, and death. Obesity is shown to increase the risk of stroke in both men and women (Shiozawa, 2021).

Type 2 diabetes

Obesity increases the risk of developing type 2 diabetes. However, losing 10% of body weight can potentially reverse it. One study found that people who dropped 10% or more of their body weight within 5 years of their diagnosis had a higher chance of achieving diabetes remission (Dambha-Miller, 2019).

High blood pressure

High blood pressure (hypertension) means that the heart has to work harder to pump blood through the arteries. Studies suggest that obesity may be the cause of high blood pressure in 65–78% of people (Shariq, 2020).

Sleep apnea

Sleep apnea is a serious condition where breathing stops and starts during the night because the upper airway is blocked. Obesity can increase the risk of sleep apnea because it adds pressure to the neck. But like type 2 diabetes, a 10 to 15 percent reduction in weight can improve sleep apnea severity (Schwartz, 2008).

While BMI is useful to some healthcare providers as an at-a-glance screening tool for health, it can't give an accurate picture of the actual amount of body fat on a person. It can't capture fat distribution on different parts of the body and does not account for factors like muscle mass, bone mass, or ethnic/racial background (Nuttall, 2015).

That's why many healthcare professionals prefer other screening tools, alone or in addition to BMI, to determine a person's risk for disease. The following factors may be used instead when determining if someone is at a healthy weight for their own body:

Waist-to-hip ratio

While BMI uses height and weight to calculate BMI, a waist-to-hip ratio (WHR) uses a comparison of waist size and hip size. Having a large amount of excess fat in the waist, as opposed to the hips, is a risk factor for developing cardiovascular disease. One study looked at over 15,000 people and found that WHR was linked to an increased risk of death, even for people with a normal BMI (Sahakyan, 2015).

Having a WHR over 1.0 is the highest risk area. In general, biological women should aim to have a WHR of 0.85 or less, while biological men should have a 0.9 or less (WHO, 2008).

Waist-to-height ratio

Like WHR, waist-to-height ratio (WHtR) calculates the risk of health problems using waist circumference in relation to height. Studies suggest that WHtR may be a better predictor of cardiovascular risk factors, like diabetes and hypertension, than BMI (Ashwell, 2014).

A healthy WHtR is one where a person's waist is less than half of their height. For example, a person who is 60 inches tall should have a waist no larger than 30 inches to be considered healthy.

Body fat percentage

Body fat percentage can be calculated using a number of different methods, including skinfold caliper tests, hydrostatic weighing, and DEXA scans. Unlike BMI, body fat testing methods allow for a more accurate measurement of the actual amount of fat on a person's body. This is helpful for athletes who have high BMIs but are actually healthy because of the low amount of body fat and higher percentage of muscle mass they carry. It's also helpful for those who may have normal BMIs but have low muscle mass. In these people, BMI may actually underestimate obesity.

No matter your weight, you are worthy of loving and caring for your body. If you have concerns about your health, make an appointment with a healthcare provider. Ro Body is a 12-month program that can help you lose up to 15% of your body weight with the help of licensed healthcare providers. 

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How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

February 08, 2022

Written by

Meagan Morris

Fact checked by

Chimene Richa, MD

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About the medical reviewer

Chimene Richa, MD

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.